A contrast ratio is a measure of a liquid crystal display (LCD) panel. When the LCD panel is in a dark room, the contrast ratio is defined as the ratio of the luminance value of the brightest color (white) to the luminance value of darkest color (black). As the brightness of the white color and the darkness of the black color are increased, the contrast ratio is increased. A high contrast ratio indicates a better image quality of the LCD panel. Nowadays, the LCD manufacturers make efforts in designing LCD panels with high contrast ratios.
FIG. 1 is a plot illustrating the relationship between the pixel voltage and the transmittance of a LCD panel. As shown in FIG. 1, a highest value of the transmittance values is observed when the applied pixel voltage is zero. As the applied pixel voltage is increased from zero to a reverse voltage (RV), the transmittance is decreased. A lowest value of the transmittance values is observed when the applied pixel voltage is equal to the reverse voltage. If the applied pixel voltage is increased from the reverse voltage, the transmittance is abruptly increased. Under this circumstance, a so-called “gray level inversion” occurs. The gray level inversion occurs in various LCD panels such as ECB (electrically controlled birefringence) mode LCD panels or TN mode+wide view film LCD panels.
Ideally, the contrast ratio is defined as the ratio of the luminance value obtained when the pixel voltage is zero to the luminance value when the pixel voltage is equal to the reverse voltage. In other words, a highest contrast ratio of the LCD panel is obtained when the maximum pixel voltage (i.e. the black voltage) is set to the reverse voltage. Due to the process variations of LCD panels, the relationships between the pixel voltage and the transmittance of different LCD panels are usually distinguished. For example, the Curve 1 has a first reverse voltage (RV1), the Curve 2 has a second reverse voltage (RV2), and the Curve 3 has a third reverse voltage (RV3). The first reverse voltage (RV1), the second reverse voltage (RV2) and the third reverse voltage (RV3) fall into a voltage range 10. For preventing from occurrence of the gray level inversion, the LCD manufacturers usually define a constant value below the lower limit of the voltage range 10 as the common black voltage of different LCD panels. In other words, there is a shift between the common black voltage and the reverse voltage. Under this circumstance, the obtained contrast ratio is not optimal.
Therefore, there is a need of providing a contrast ratio improving device and a contrast ratio improving method so as to obviate the drawbacks encountered from the prior art.